This invention relates to apparatus for restoring the cathode emission characteristics in cathode ray tubes having multiple guns and, in particular, relates to apparatus for testing, rejuvenating, and cleaning three gun, color television cathode ray picture tubes.
It is well known in the field of color television that after a picture tube has been used for an extended period of time, the rate at which electrons are emitted from the cathodes of one or more of the electron guns decreases sufficiently that the tube no longer produces an acceptable picture. The picture tube is, of course, one of the most expensive components in a color television receiver. The desirability of restoring the emission characteristics of the cathodes in color television picture tubes has been recognized in the past since such restoration would prolong the useful life of the picture tube. Various apparatus have been provided to perform this restoration; U.S. Pat. No. 3,641,391 to Albert R. Badewitz dated Feb. 8, 1972, and entitled "Device for Restoring Cathode Emission in Cathode-Ray Tube Guns," purports to described one such apparatus. In the restoration apparatus previously provided it has been known to provide circuitry for supplying a relatively high potential alternating voltage between the cathodes and the control grids of the cathode ray tube electron guns in order to effectuate a process referred to as "cleaning" the cathodes, and also circuitry for supplying a relatively high potential direct voltage between the cathode and the control grids of the cathode ray tube electron guns in order to effectuate a process referred to as "rejuvenating" the cathodes. The rejuvenation process is thought to result in the deposit of new emissive material on the electron emitting surfaces of the cathodes. However, the rejuvenation process may result in the cathode becoming polarized or the new emissive material being unevenly deposited across the emitting surface. The cleaning process is to cure these problems. Moreover, a certain amount of rejuvenation may also occur during the cleaning process. It is usually not desirable to clean or rejuvenate the cathode of an electron gun which does not require it as the application of high potentials across the cathode and control grid may on occasion cause permanent damage to the tube by, for example, shorting those elements together or partially destroying the layer of electron emitting material on the cathode. It has also been found desirable in the past to combine with apparatus for restoring cathode emission apparatus for testing the picture tube for the presence of defects in the tube other than low cathode emission rates.
However, the apparatus provided prior to this invention have had one or more of a number of drawbacks associated therewith. For example, in some such apparatus if one desired to clean or rejuvenate one of the electron guns in a multiple gun cathode ray tube, he had no choice but to perform the cleaning and/or rejuvenation process on all the electron guns simultaneously. In some such apparatus during the cleaning process, no indication was given to the operator of whether a short existed between the cathode and control grid of one or more of the guns. If such an indication were given during the cleaning process, the operator would then know that rejuvenation of the shorted gun or guns should not be attempted; this indication is particularly desirable where the potential used for rejuvenation is signficantly greater than that used for cleaning and/or the potential source used for rejuvenation is capable of delivering much higher currents than the source used for cleaning. Further, in apparatus available previous to this invention there was no circuitry for separately determining just which of the electron guns in a multiple gun cathode ray tube required cleaning or rejuvenation. In some apparatus having the circuitry just referred to, it way insufficiently versatile to be used with a wide range of types of cathode ray tubes and, in particular, could not be used with color television cathode ray picture tubes of the type known as "in-line" tubes and having the control grids of all three electron guns connected together in parallel. Further, prior restoration apparatus often did not provide circuitry for testing important parameters of the cathode ray tube and were often inconvenient or cumbersome to operate, some to the point of being dangerous.